The present invention relates to nuclear magnetic resonance magnetic resonance (NMR) imaging methods, and more particularly, to a high throughput method for obtaining in the course of a single scan T1-weighted and T2-weighted NMR image data for a plurality of selected planes in an object, thus providing significantly shorter NMR scanning times, and doing so in as convenient and efficient manner as possible.
By way of general background, the nuclear magnetic resonance phenomena occurs in atomic nuclei having an odd number of protons and/or neutrons. Due to the spin of the protons and neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a homogeneous magnetic field, a greater number of nuclear magnetic moments align with the direction of the magnetic field to produce a net macroscopic magnetization in the direction of the field. Under the influence of the magnetic field, the magnetic moments precess about the axis of the field at a frequency which is dependent upon the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, .omega., also referred to as the Larmor frequency, is given by the equation .omega.=.gamma.B in which .gamma. is the gryomagnetic ratio (which is a constant for each particular atomic nuclei) and in which B is the strength of the magnetic field acting upon the nuclear spins.
The orientation of magnetization M can be perturbed by the application of a magnetic field oscillating at the Larmor frequency, which has the effect of rotating the magnetization away from the direction of the static field. Typically, the oscillating magnetic field is applied in a direction orthogonal to the direction of the static magnetic field by means of a radio frequency (RF) pulse through coils connected to a radio frequency transmitting apparatus. In essence, the net magnetic vector or orientation of magnetization M is rotated away from the direction of the static field. One typical RF pulse is that which has either sufficient magnitude or duration to rotate the magnetization M into a transverse plane (i.e. 90.degree. from the direction of the static field) and is thus known as a 90.degree. RF pulse. Similarly, if the magnitude or duration of the RF pulse is selected to be twice that of a 90.degree. pulse, the magnetization M will change direction 180.degree. from the main or static magnetic field.
When the RF perturbing pulse is stopped (by turning the RF transmitter off), the nuclear spins tend to slowly realign or relax back to the equilibrium position, and emit an NMR signal which can be detected with an RF receiver coil (which may, and often does, comprise the same coil used as the transmitter). The signal emitted is dependent on three basic parameters--namely, the density of the excited nuclei, the spin-lattice relaxation time (T1) and spin-spin relaxation time (T2), the latter two parameters both being exponential time constants which characterize the rate of return to equilibrium of the longitudinal and transverse magnetization components following the application of the perturbing RF pulse. These NMR parameters of spin density, T1 and T2 are related to the atomic nuclei subjected to the NMR phenomena.
Since the discovery and recognition by Damadian in the early 1970's that nuclear magnetic resonance techniques can be used to scan the human body to yield useful diagnostic information, and the subsequent development of medical NMR imaging apparatus for obtaining NMR images of the internal structures of patients, much work and effort has been expended to improve and refine the techniques for obtaining NMR images, as well as determine the diagnostic usefulness of NMR images. As a result, NMR imaging, or magnetic resonance imaging as it is sometimes known, has today proven to be an extremely useful modality to the medical community for the purposes of detecting abnormalities within the body and the subsequent diagnosis thereof. In this regard, it is known that variations in the relaxation times T1 and T2 are more closely associated with the differences between healthy and diseased tissue, and thus, from a diagnostic viewpoint, images which display or show significant T1 and/or T2 contrast have proven to be of greater diagnostic interest than those showing and displaying differences in, for example, proton density.
In the application of NMR principles to medicine and medical diagnostic imaging of live human subjects, NMR signals are obtained for a multitude of small areas in a patient, known as picture elements or pixels, and then used to construct an image or pictorial representation of a particular area of the patient being examined. More particularly, the intensity of the NMR signals are measured for the multitude of pixels, such intensity of each signal being a complex function of the tissue-related parameters used in gathering the image information. These tissue-related parameters are the spin-density of the particular nuclei being imaged (usually protons or hydrogen atoms in most medical NMR imaging applications) as well as the T1 and T2 relaxation times. For convenience of physicians and other medical personnel, the various intensity values for each of the pixels are displayed on a gray scale, in which the individual intensity values are merely a visual translation into the gray scale of the intensity values measured during the imaging scan or procedure.
In order to construct images from the collection of NMR signals, present day NMR imaging apparatus and equipment generally utilize magnetic field gradients for selecting a particular slice or plane of the object to be imaged and for encoding spatial information into the NMR signals. For instance, one conventional technique involves subjecting an object to a continuous static homogeneous field extending along a first direction and to sets of sequences of orthogonal magnetic field gradients which each generate a magnetic field component in the same direction as the static field but whose strengths vary along the direction of the gradients. In accordance with this known technique, nuclear spins in a selected plane are excited by a selective RF pulse in the presence of one of the magnetic field gradients, the frequency of the selective RF pulse corresponding to the Larmor frequency for only the selected plane of the object as determined by the magnetic field gradient imposed on the static magnetic field. Conveniently, the applied magnetic field gradient is designated the slice selector gradient. The selected plane will thus extend in a direction perpendicular to the gradient direction of the slice selector magnetic field gradient. The excited selected spins are then subjected to the other magnetic field gradients (which can be designated the read out and phase encoding magnetic field gradients), utilizing a plurality of repetitions in which the amplitude of the phase encoding gradient is varied for each repetition and in which the read out gradient is applied during the reading out of the generated NMR signals. The received NMR signals are then transformed utilizing conventional two-dimensional Fourier transform techniques. The read out magnetic field and phase encoding magnetic field gradients serve to encode spatial information into the collection of NMR signals so that two-dimensional images of the NMR signals in the selected plane can be constructed. As will be appreciated, during the scanning sequence, the various magnetic field gradients are repeatedly switched on and off at the desired intervals. Such a two-dimensional Fourier transform imaging technique and the pulse sequence for such a technique is described in the book entitled "Nuclear Magnetic Resonance Imaging in Medicine", published in 1981 by Igaku-Shoin, Ltd., Tokyo, and is sometimes known as spinwarp imaging.
Furthermore, many NMR imaging schemes today rely on the collection of spin-echo NMR signals rather than free induction decay (FID) signals. The FID NMR signals are achieved by application of a 90.degree. RF excitation pulse and then reading out of the produced signal. In utilizing spin-echo signals, a 90.degree. RF excitation pulse is followed by the application of a 180.degree. rephasing RF pulse at a predetermined time interval after the 90.degree. pulse. This produces a spin-echo signal at a corresponding time interval after the application of the 180.degree. RF pulse. In NMR parlance, the time of the produced spin-echo NMR signal after the 90.degree. RF excitation pulse is designated as TE (for time of echo). Thus, the 180.degree. RF pulse is applied at a time interval of TE/2 after the 90.degree. RF pulse.
Depending upon the NMR pulse sequences utilized in obtaining the collection of NMR signals for the various pulse sequences, the relative contribution of the different NMR parameters to the measured intensity values, and thus the NMR image, can be emphasized or deemphasized. For example, techniques are known for producing T1-weighted and T2-weighted NMR images, which are images which emphasize the T1 contributions and the T2 contributions, respectively. In this regard, T1-weighted and T2-weighted images each have different information content and have each proven useful in connection with making a diagnosis of a patient.
In a T1-weighted image, the contribution provided by the T1 parameter is emphasized which minimizes the contribution provided by the T2 NMR parameter. Thus, in connection with a T1-weighted image, the echo or data acquisition time after application of the NMR excitation pulse is chosen to be relatively small so as to minimize the effects of dephasing which is related to the T2 relaxation time. Also, the repetition interval between application of the excitation pulses is selected so as to be neither too short (to prevent complete saturation) nor too long (to prevent complete equilibrium), and thus is generally chosen to correspond to the T1 values likely to be present in the patient. A typical time interval between application of the excitation pulse and reading for a T1-weighted image is 28 milliseconds, and a typical repetition rate is between 350-600 milliseconds.
On the other hand, in connection with a T2-weighted scan, the time interval from the application of the excitation pulse to the time the data is taken is preferrably on the order of the T2 times likely to be present in the patient so as to create an appreciable difference between the nuclei population by the T2 decay process. Further, the repetition time between excitation pulses is significantly longer than the expected T1 values of the patient so that the excited nuclei will relax to their ground state before application of a subsequent excitation pulse and data acquisition. Typical values for the time interval from the initial application of the excitation pulse to data acquisition and for the repetition times for T2-weighted images are 56-112 milliseconds and 1,500-3,000 milliseconds, respectively.
Further in this regard, one of the major problems in medical NMR imaging is patient throughput. Thus, numerous efforts have been devoted to the development of techniques for obtaining images in a shorter period of time. For example, typical acquisition times for obtaining T1-weighted images are on the order of four minutes, whereas acquisition times of seventeen minutes are typical for T2-weighted images. Although various efforts have been devoted to the development of techniques for shortening the scan times, to date, they have generally resulted in a sacrifice of the diagnostic quality of the information obtained, and thus, have not yet proven satisfactory.
Still further in this regard, several techniques have been developed for producing a plurality of NMR images during the course of a single scan. For instance, multi-echo pulse techniques have been developed for the generation and collection of NMR signals from different produced echoes in the course of a single scan, which can then be used to produce several images in the same plane of the object, some of which may correspond to T2-weighted images. However, with such multi-echo techniques, although the TE time interval can be varied, the repetition rate is the same for each produced image, thus precluding the generation of truly T1-weighted images.
Also, U.S. Pat. No. 4,549,140 is directed to an NMR imaging method utilizing combined, interleaved pulse sequences for generation of computed T1 and T2 maps or images based upon sets of NMR signals at different repetition times but the same TE (or multiple of TE) time intervals for data acquisition. Such computed T1 and T2 images are not T1-weighted and T2-weighted images, however. In particular, T1 and T2-weighted images are images which are based upon a combination of the three NMR parameters of proton density, T1 and T2 but in which the T1 and T2 parameters, respectively, are emphasized to a greater extent than the other parameters, whereas computed T1 and T2 images are only dependent on, respectively, the T1 and T2 parameters alone. Furthermore, the techniques disclosed in such patent are only directed to obtaining images in a single plane of the object, and not to obtaining a plurality of images for a multitude of different planes in a manner so that the total imaging time is less than that in which the image data is obtained separately.
The technique of multi-slice imagiing has been developed for obtaining NMR images from a multiple number of planes or regions of a patient in which the nuclei in a plurality of planes or regions of the patient are excited and NMR signals read out therefrom for different regions or slices of the object during a single scan. More particularly, in multi-slice imaging, the slices or planes in the imaging volume are excited one after another during different portions of the interval between repetitions by packing an integral number of slice excitations between successive excitations in one particular plane or slice. For example, when selective RF pulses are applied in the presence of a magnetic field gradient, only a limited region of the patient is excited due to satisfaction of the resonance conditions. Accordingly, different frequencies will excite different parts of a patient. As the repetition sequence for any particular slice involves an excitation followed by reading of the produced signal and then followed by a recovery interval before applying the excitation pulse in a subsequent repetition, the nuclei in differing regions or planes can be excited during the recovery interval for one particular plane, thus efficiently utilizing the recovery time interval to selectively excite nuclei and read out NMR signals in other planes. Generally, the number of planes for which NMR images can be obtained is dependent on the recovery time interval between successive excitation pulses in a single plane and the sequence interval required for exciting and reading out of an NMR signal in one plane plus the time for switching of the gradients. For example, in connection with a spin-echo imaging sequence, the slice interval will correspond to the time necessary to apply a 90.degree. excitation RF pulse, the apply a 180.degree. rephasing RF pulse, to observe the echo produced thereby and to raise and lower the appropriate gradients. During the portion of a repetition sequence following the sequence time interval, additional selected planes can be sequenced utilizing different frequencies in a consecutive manner.
In accordance with present day techniques, T1-weighted and T2-weighted images are generally obtained in two separate scans, each being conducted under different pulsing repetition rates. Also, present day techniques generally require operator intervention between the respective scans. Even in connection with techniques utilizing sequence queing software for minimizing operator intervention, still there is no significant gains in patient throughput. Further, even in connection with multi-slice acquisition techniques, the number of slices or planes imaged in connection with a single scan for obtaining T2-weighted images generally are significantly higher than the number of slices or planes imaged in connection with T1-weighted scans, and therefore, additional scans must be obtained in order to obtain a corresponding number of T1-weighted images. More particularly, in connection with a T2-weighted imaging scan in which the repetition interval is 2,000 milliseconds, and in which the sequence interval for applying the 90.degree. pulse, 180.degree. pulse, to observe the produced echo and the time needed to raise and lower the appropriate gradients is on the order of 100 milliseconds, there is room for applying consecutive pairs of pulses and readings during the repetition sequence at twenty different frequencies and thus obtain imaging data for twenty different planes. On the other hand, in connection with a T1-weighted scan in which the sequence interval is on the order of 50 milliseconds and the recovery interval is on the order of 500 milliseconds, there is only room for ten slices.
Consequently, in conventional techniques for obtaining both T1-weighted data and T2-weighted data, not only are separate scans required for obtaining T1-weighted and T2-weighted images, but further, additional T1-weighted scans are required to obtain T1-weighted images for the same number of slices for which T2-weighted scans are obtained, thereby increasing the number of scans of the patient which must be performed and the time necessary to complete such scans. Here, it should also be appreciated that between each imaging scan which is performed on a patient, it is generally necessary to allow the patient to rest. Also, a certain amount of time is necessary when conducting a scan for operator setup, loading of information into the apparatus respecting the conditions and sequencing for collection of data, etc. Therefore, in order to obtain, using conventioal techniques, T2-weighted images for a plurality of planes and a corresponding number of T1-weighted images, the total scanning time is quite high, typically on the order of forty-five minutes.
Accordingly, it will be appreciated that a significant need exists for shortening the period of time for obtaining the T1-weighted and T2-weighted images, and in particular, to reduce the total acquisition time for acquiring the data and information from which T1-weighted and T2-weighted NMR images are constructed. That is, in connection with multi-slice acquisition techniques, with present day methods, variable repetition times and variable echo times can only be obtained by rescanning of a patient.